1. Field of the Invention
An object of the present invention is a method for the acquisition of images with a gamma camera during a tomographic examination in nuclear medecine.
The invention is especially promising when the self-attenuation of the gamma rays in the patient is high: in particular for myocardial tomography, hence for organs located on the side of the human body. However, this in no way restricts the invention to this type of use.
2. Description of the Prior Art
Gamma cameras are described, for example, in the U.S. patent by Anger, U.S. Pat. No. 3,011,057. A gamma camera is an apparatus comprising a stand, which is rotating, fixed or movable with respect to the ground and carries a detector, also called a detector head, at the end of an arm. This detector is provided with an array of photomultiplier tubes, the input faces of which are juxtaposed with one another and constitute the detection surface of the detector head and its detection field.
The following is the principle of the examination. A radioactive substance is injected into a patient to be examined. This substance is, for example, thallium for the examination of the myocardium. The radioactive emission excites a scintillator crystal of the detector which converts the energy of the gamma photons into a light energy that can be detected by the photomultiplier tubes. The scintillator crystal is preceded, in a standard way, by a collimator defining a sighting direction and characterized by a focal point. This focal point is pushed back to infinity in the case of collimators with parallel, straight or inclined holes. The focal point is at a finite distance, which is positive or negative, in the case of convergent or divergent collimators. The focal point may be off-centered with respect to a central sighting direction.
The scintillations emitted are detected by the photomultiplier tubes which produce electrical signals depending on the light intensity received. By carrying out barycentric tracking operations on all these electrical signals, it is possible, in a known way, to determine the localization X Y of the origin of the scintillation in the detection field. An incremental acquisition is then carried out by totalizing the number of scintillations (or strokes) detected per localization element called a pixel.
By leaving the detector head in a given position for a certain time above the examined body, it is then possible, for a given angle of sight, called a projection, to obtain an image that reveals the concentration of the emitting substance in the body. A tomographic examination consists in acquiring one image per angle of sight, for a large number of angles of sight, evenly spaced out on an angular sector of at least 180.degree.. It is then possible, with computation algorithms, notably filtered back projection, to reconstitute the image of a volume of the body.
For cardiac applications, given the movement of the heart, the procedure furthermore entails a synchronization of acquisitions. To increase the sensitivity of the camera, the common practice has been to use a rotating stand provided with two detector head instead of only one. These two heads face each other and rotate, together, about the patient being examined. They both contribute to the acquisition of the projections. The sighting directions of the two detectors then coincide. They go through the rotation axis of the system.
This type of geometry does not improve the sensitivity for cardiac examinations because of the following reasons. The attenuation is all the greater as the energy of the isotope used is low. As a consequence, the projections of the 180.degree. angular sector acquired (which gives the nearest projection of the organ to be studied and minimises the self-attenuation of the tissues interposed between this organ and the detector) are far more significant than the projections of the opposite angular sector. Furthermore, the use, if any, of the opposite sector during the filtered back projection lowers the value of the result obtained. It thus appears that the second detector is unnecessary in this case.
This drawback, moreover, is also found with three-head or four-head gamma cameras, for which there are always one or two unnecessary detector heads.
It is an object of the invention to overcome the drawbacks mentioned by proposing, with a two-head camera, a camera geometry and kinematic parameters of examination that are different so as to double the sensitivity. The principle of the invention consists in off-setting the sighting center, defined as the intersection of the two sighting directions, with respect to the axis of rotation of the gamma camera. This may be obtained, for example, in two ways. In a preferred way, the detection fields of the two detector heads are no longer parallel. This geometry is obtained by (preferably) symmetrical angular rotations or "angulations" of the detector heads with respect to the horizontal plane passing through the rotation axis of the stand while this stand is in a vertical orientation. In another way, collimators with inclined holes make it possible to obtain the offsetting of the sighting center with respect to the axis of rotation, while the detection fields may remain parallel. In this case, it is necessary to have as many collimators as there are possible offset values. Naturally, it is possible to associate the two techniques and obtain the chosen offset, by the angulation of the heads on the one hand and by providing the detectors with inclined hole collimators on the other.
It is then shown that the rotation angle about the body can be divided by two. In particular, in a preferred variant of the invention, when the angulation of the detectors or the sighting direction of the collimators is equal to 45.degree., the 180.degree. tomographic acquisition is obtained by a rotation of the stand of only 90.degree.. The patient supporting element is then driven by an ascending motion while the stand is driven by a motion of lateral translation (or vice versa), in synchronism with the rotation of the detectors. The relative motion of the patient with respect to the stand or of the stand with respect to the patient being an arc of a circle.